This application relates to substrate processing equipment used for substrates such as semiconductor wafers. In particular, this application addresses tuning of process modules used to process such substrates.
A process module is used to carry out a particular process step, or series of steps, on a substrate during manufacture of products including integrated circuits and flat-panel displays. Examples of process modules used in the semiconductor industry include etch chambers, rapid thermal processing chambers, chemical vapor deposition chambers, physical vapor deposition chambers, furnaces, chemical mechanical polishing systems and hotplates. These process modules process silicon wafers of various sizes including 200 millimeter and 300 millimeter wafers. Gallium Arsenide and other semiconductor substrates may also be processed. Similar process modules are used to process other substrates including flat panel displays.
Process modules may be tuned by processing test substrates with differing process inputs and analyzing the results on the test substrates. This provides a form of feedback shown in FIG. 1. A wafer is first processed and then the results of processing are measured in a metrology tool and the process module is adjusted according to the data obtained. However, this method of tuning is expensive and time-consuming. Many test substrates may be needed and additional metrology equipment is necessary to make measurements. Some process modules may not be easily tuned in this way because some of the process conditions may affect the process in ways that are not easily measurable, or are only measurable after more process steps are performed. For example, the effect of a hotplate used for post exposure bake of a photoresist layer on a substrate may be discernable only after the substrate has been processed further and the photoresist layer has been developed. This method gives only indirect information on the process conditions in the process module. Temperature in the process module may be inferred from characteristics of the resulting layer but this may not be accurate because of other factors that affect the same characteristics. Thus, using indirect information from test substrates is of limited value.
Another method of tuning a process module is shown in FIG. 2. This shows a process condition measuring device placed in the process module providing data to a base station. A process condition measuring device (PCMD) may be made to resemble the substrates that are normally handled by the process module. Thus, for use in a process module configured for 300 millimeter wafers, a PCMD might resemble a 300 millimeter wafer. A PCMD has sensors on its surface or within cavities so that process conditions may be measured. These measurements approximate the actual process conditions experienced by a regular substrate during production. A PCMD may transmit data through a cable to a base station, or transmission may be wireless. A cable may affect heat flow and other characteristics of the substrate. Wireless transmission does not require a cable and therefore the PCMD may be made to more closely resemble a regular substrate. Transmission of data may be in real-time or at any time after data is collected. Data may be stored in a memory and later retrieved when the PCMD is outside the process module. This may be more practical for process modules that have many potential sources of electromagnetic interference such as RF power supplies and microwave generators. With a PCMD, one or more process condition may be directly measured instead of being inferred from process results. For example, to tune the temperature in a process module, temperature is directly measured by a PCMD. Examples of PCMDs are provided in U.S. Pat. Nos. 6,190,040 and 6,325,536, which patents are hereby incorporated in their entirety.
Modern process modules may be complex, having many process conditions that affect the quality of the substrate that is processed. Process conditions such as temperature, pressure, gas flow rate, chemical concentration, plasma density, etch rate and deposition rate may be affected by process inputs including the power provided to heating units, temperature setpoint, pumping speed, pressure setpoint, geometry of the module, substrate position, gas flow provided, radio-frequency power etc. The interactions of these process inputs to provide the process conditions may be complex and difficult to tune to obtain a desired result.
In addition, a particular process condition such as temperature or etch rate may vary across a substrate. Thus, one portion of the substrate may have a process condition at a desired level while other portions of the same substrate do not have the process condition at the desired level. Typically, for a process to be tuned, a wafer must have process conditions within a desired range across a primary surface. For a 300 millimeter wafer, process conditions may need to be substantially uniform across the surface in order to produce integrated circuits of adequate quality. Typically, a portion of the primary surface, close to the edge of the wafer does not need to be within the desired range because this portion is not used.
Feedback controllers such as Proportional, plus Integral, plus Derivative (PID) controllers are widely used to maintain a process condition in a process module within a specified process window. A PID controller provides a modified process input to the process in response to a difference between a measured process output and a desired process output (or error). The equation used by a PID controller is given by:CO=Kp·E+Ki∫Edt+Kd·dE/dt  (Eqn. 1)where CO is the controller output, or the process input that is controlled. The equation has three terms, a proportional term, an integral, term and a derivative term. The proportional term (Kp×E) is proportional to the error E. A proportional constant, or gain (Kp), determines how great a response is produced by a particular value of E. Thus, a high gain produces a greater response for a given difference. An integral term (Ki∫E·dt) produces a long-term corrective change by integrating the error E over time. A constant called Ki determines the magnitude of the correction. A derivative term (Kd×dE/Dt) produces a response that depends on the rate of change in the error. This term is proportional to a constant Kd. Other controllers do not have a derivative response and so are PI controllers. Other feedback controllers using different equations are also used. Controllers may also operate without feedback such as by maintaining a fixed output.
Tuning controlled systems may require adjusting one or more setpoints. Sometimes, an offset feature is present that allows a setpoint for a controlled system to be offset by a particular amount. Tuning a simple controlled system may be a matter of setting an offset to the right value. Tuning a controlled system to achieve a desired dynamic behavior may require changing other operating parameters such as PID constants to ensure that the process input responds rapidly enough, but avoids overshoot or oscillation. Maximum and minimum power settings are other operating parameters that may be used to regulate the response of the controller. Setting maximum and minimum power levels keeps the output in a limited range and thus prevents large oscillations or overshoot.
To tune a process module for a particular process condition it is generally necessary to make several adjustments to process inputs of the process module and note the effect on a process condition. For example, tuning a multi-zone hotplate may require making adjustments to the heat supplied to several of the zones. Where a PCMD is present in the process module, it may be possible to monitor the effect of such adjustments in real-time. However, tuning is generally directed to achieving temperature uniformity in a steady-state condition. Therefore, when an adjustment is made, it may be necessary to wait for the system to reach a new equilibrium before the next adjustment is made. Depending on the number of adjustments needed, this may be quite time-consuming. Each adjustment may affect more than one process output. Adjusting the heat supplied to a heating zone affects the temperature not only in that zone, but in neighboring zones also because heat is conducted to the other zones. Therefore, as each zone is tuned in turn, neighboring zones that may already have been tuned may be affected so that they are no longer tuned.
In some facilities, multiple process modules may operate in parallel to perform the same function. For example, two process modules might do the same etch step, with half the wafers going to one module and half to the other. It is important that there are no significant differences between the substrates from the modules so that all devices from all substrates have similar characteristics. Tuning may be necessary to match such process modules and achieve module-to-module uniformity. For example, hotplates may be tuned so that different hotplates have the same temperature profile. This is in addition to tuning individual hotplates to have a particular temperature profile across the hotplate. Typically, the goal is to produce the same conditions at all points of all substrates.
Trends in technology make tuning of process modules more important. Substrates are generally increasing in size. For example, the semiconductor industry moved from 150 millimeter to 200 millimeter wafers and is now moving to 300 millimeter wafers. Flat-panel display substrates may have dimensions in excess of 2 meters. Uniformity becomes harder to maintain over such large substrates. As substrates have increased in size, the dimensions of the features on those substrates have decreased in size. These small features sometimes require process conditions to be controlled within very narrow windows. For example, chemically amplified photoresist is extremely sensitive to the temperature of the post exposure bake step. Variations of less than 1 degree Centigrade may affect the Critical Dimension (CD) of the photoresist pattern. In addition, such processes may be sensitive to transient conditions as well as steady-state conditions.
Therefore, there is a need for a method of tuning a multi-input, multi-output process module to provide control of process conditions within narrow process windows. Furthermore, there is a need for a method of tuning that is rapid and may be implemented on existing process modules with little disruption to the module.